It was discovered that ICU did not properly handle invalid locale data
during Unicode conversion. If an application using ICU processed crafted
data, an attacker could cause it to crash or potentially execute arbitrary
code with the privileges of the user invoking the program.

Update instructions:

The problem can be corrected by updating your system to the following
package versions:

Ubuntu 11.10:
libicu44 4.4.2-2ubuntu0.11.10.1

Ubuntu 11.04:
libicu44 4.4.2-2ubuntu0.11.04.1

Ubuntu 10.10:
libicu42 4.2.1-3ubuntu0.10.10.1

Ubuntu 10.04 LTS:
libicu42 4.2.1-3ubuntu0.10.04.1

In general, a standard system update will make all the necessary changes.